gpio: davinci: fix gpio selection for OF

The driver missed an of_xlate function to translate gpio numbers
as found in the DT to the correct chip and number.

While there I've set #gpio_cells to a fixed value of 2.

I've used gpio-pxa.c as template for those changes and tested my changes
successfully on a da850 board using entries for gpio-leds in a DT. So I didn't
reinvent the wheel but just copied and tested stuff.

Thanks to Grygorii Strashko for the hint to the existing code in gpio-pxa.

Signed-off-by: Alexander Holler <holler@ahsoftware.de>
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/gpio/gpio-davinci.c

@@ -174,6 +174,27 @@ of_err:
 	return NULL;
 }
 
+#ifdef CONFIG_OF_GPIO
+static int davinci_gpio_of_xlate(struct gpio_chip *gc,
+			     const struct of_phandle_args *gpiospec,
+			     u32 *flags)
+{
+	struct davinci_gpio_controller *chips = dev_get_drvdata(gc->dev);
+	struct davinci_gpio_platform_data *pdata = dev_get_platdata(gc->dev);
+
+	if (gpiospec->args[0] > pdata->ngpio)
+		return -EINVAL;
+
+	if (gc != &chips[gpiospec->args[0] / 32].chip)
+		return -EINVAL;
+
+	if (flags)
+		*flags = gpiospec->args[1];
+
+	return gpiospec->args[0] % 32;
+}
+#endif
+
 static int davinci_gpio_probe(struct platform_device *pdev)
 {
 	int i, base;
@@ -238,6 +259,9 @@ static int davinci_gpio_probe(struct platform_device *pdev)
 			chips[i].chip.ngpio = 32;
 
 #ifdef CONFIG_OF_GPIO
+		chips[i].chip.of_gpio_n_cells = 2;
+		chips[i].chip.of_xlate = davinci_gpio_of_xlate;
+		chips[i].chip.dev = dev;
 		chips[i].chip.of_node = dev->of_node;
 #endif
 		spin_lock_init(&chips[i].lock);